Investigate to use go mod in Travis CI

[shu-mutou]

npm ci seems to update go.mod file to use latest.
To use go mod, we need to investigate more.

BTW, Error about src/app/backend/dashboard.go:158:26: undefined: prometheus.Handler is caused due to prometheus.Handler() was removed in latest (1.0.0) prometheus/client-golang.

[eloyekunle]

In the package.json postinstall, I think we should download a specific version of golangci-lint (via curl), instead of via go get. Coincidentally, that's what's recommended by the golangci-lint team.

@shu-mutou Our prometheus/client_golang is currently fixed at v0.9.2 (here). If npm ci is updating the version during the build, then #3938 should fix it.

[shu-mutou]

I understood we should not use go get to install tools.
In the future, if we would want to add the other tools written in golang, we may meet same problem.
Can we move export GO111MODULE=on just before npm ci? If so, I think we can use go get before setting GO111MODULE.
Also, should we set GO111MODULE in our local environments?

[maciaszczykm]

@shu-mutou https://github.com/kubernetes/dashboard/blob/master/.travis.yml#L20 is already before npm ci I think.

[shu-mutou]

Oh, yes! You're right!! We can use go get before it. 😅